This invention is an improved version of the encoder(s) disclosed in U.S. Pat. Nos. 5,559,600; 5,486,923 and 5,646,730 assigned to MicroE, Inc. of Natick, Mass., the assignee of the subject application. It optimizes the reflective mode operation of the disclosed encoders by reducing the previous system""s sensitivity to grating roll. The invention is directly applicable to other reflective, grating based position sensing encoders, including encoders that do not use the technology disclosed in the foregoing referenced patents.
This invention incorporates a second wavefront compensator and a second detector array in parallel with the original wavefront compensator and detector array in a reflective diffractive encoder. No additional grating is required. This second optical channel provides for the measurement of an additional degree of freedom (roll), the effects of which can then be removed from the main measurement channel.
The core principle used in this invention is the creation of a second grating-movement sensing channel in which the relative effects of grating roll and grating displacement are different in this second channel when compared to the first sensing channel. As is described in this disclosure, we create this differentiation by having different ratios of the grating period to the detector fringe period in the two channels.
This invention is novel in its ability to independently measure two degrees of freedom (roll and translation) of a single moving grating in a reflective, diffractive optical encoder. In many applications the benefit is to remove the effects of roll from the translation measurement, but in other applications it may be desirable to use this sensor for its ability to measure roll with interferometric accuracy.
The above and other features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of various embodiments of the present invention and the accompanying drawings.